The discovery of the planet, called GJ 1214b, represents a "major step forward" in the search for worlds similar to the Earth, says the Professor Geoffrey Marcy of University of California, Berkeley in a commentary also in Nature.

The "newfound world" is too hot to sustain life as we know it, say researchers.

Its density suggests however it "is composed of about three-fourths water and other ices and one fourth-rock," they say. "There are also tantalising hints that the planet has a gaseous atmosphere."

Its temperature is estimated at between 280°C and 120°C with its host star about one-fifth the size of the Sun, according to the scientists.

"Despite its hot temperature, this appears to be a waterworld," says Zachory Berta, a graduate student who first spotted hints of the planet's presence.

"It is much smaller, cooler and more Earth-like than any other known exoplanet," he says.

An exoplanet is one outside of our solar system.

Less dense than rock

Berta says some of the water would likely be in crystalline form that exists at pressures greater than 20,000 times Earth's sea-level atmosphere.

Its temperature is much lower than that on the only similar discovery, called CoRoT-7b, which revolves around a star that is much hotter, the scientists say.

The CoRoT-7b has a density close to that of the Earth (5.5 grams per cubic centimetre) and seems to be rocky, while the new discovery appears to be much less dense (1.9 g/cm3).

"To keep the planet's density that low requires that it contains large amounts of water," says Marcy. "It must contain a huge amount of water, roughly 50% by mass."

The "wild card" is the make-up of the gases on the planet, he says.

The planet orbits every 38 hours around a small, faint star that was first spotted by eight ground-based telescopes no larger than those used by amateur astronomers.

Its relative closeness makes it possible to study it to determine its atmosphere.

Professor David Charbonneau, who led the research team says, "That will make it the first super-Earth with a confirmed atmosphere - even though that atmosphere probably won't be hospitable to life as we know it."